NO, 5 MIDDLE CAMBRIAN ANNELIDS III 



CAMBRIAN ANNELIDS. 



I have often searched the fine shales of the pre-Cambrian and 

 Cambrian strata for remains of annehds but it was not until the 

 summer of 1910 that anything more than trails and borings were 

 found. 



The annelids of the Burgess shale, like the holothurians and 

 Medusse, are pressed flat so that the animal is represented by only 

 a thin film. Fortunately this is darker than the shale and usually 

 shiny, and the contents of the animal are often preserved as a glisten- 

 ing silvery surface, even to the fine details of structure. How 

 clearly the specimens exhibit both external and internal characters 

 is shown by the plate figures which are reproduced from photo- 

 graphs made by reflected light. As it was impossible to bring out 

 all the characters through light falling from one direction, the photo- 

 graphs were touched up by pencil, but not to such an extent as to in- 

 troduce interpretation of structure not shown by the fossil. 



ClassiUcation. — I have followed very largely the classification of 

 Parker and Haswell's Text-Book of Zoology, Vol. i, London, 19 10. 



The Class Chaetognatha is represented by one genus and species, 

 Amiskwia sagittif ormis . The Class Chgetopoda by six genera of the 

 sub-class Polychaeta as follows : Miskoia, Aysheaia, Canadia, 

 Worthcnclla, Pollingcria, IVizvaxia, and Sclkirkia; and the Class 

 Gephyrea by four genera : Ottoia. BarifHa, Pikaia, and Oesia. 



The list of families, genera, and species may be found in the table 

 of contents. 



Relations to living annelids. — The discovery of this remarkable 

 group of annelids in the Burgess shale ' member of the Stephen 

 formation opens up a new point of view on the development of the 

 Annulata. The fact that from one very limited locality there have 

 been collected eleven genera belonging to widely separated families 

 points clearly to the conclusion that the fundamental characters of 

 all the classes had been developed prior to Middle Cambrian time. 

 No examples of the Class Hirudinea have been recognized, but the 

 segmentation of the Ch?etopoda is present in Ottoia and BanfUa, anne- 

 lids which otherwise are true Gephyreans. To a certain extent these 

 two genera serve to link the Chaetopoda and Hirudinea. 



I should not be at all surprised to find representatives of the 

 Archi-Annelida in the Burgess shale. Thus far the annelids col- 

 lected were incidental to other fossils rather than a direct object of 

 search. 



^Smithsonian Misc. Coll., Vol. 57. No. 3, 191 1, p. 51. 



